Cathode ray tube (CRT) display technology has been widely used for several decades. The main advantages of CRT displays are their excellent display performance and their low price. However, the large size and high operating voltage of CRTs makes them unsuitable for some applications.
In recent years, various flat panel displays has been developed, particularly with a view to achieving the same display performance as CRT displays. A liquid crystal display (LCD) panel is one type of flat panel display device that can be readily mass-produced. The lightness and slimness of LCD panels make them suitable for a wide variety of uses, such as in personal digital assistants (PDAs), notebook personal computers, and other information appliances.
Because the liquid crystal of an LCD panel cannot emit light by itself, in general, a backlight module needs to be employed in the LCD panel. Many key characteristics for evaluating the performance of an LCD display depend on the backlight module employed. In particular, high illumination performance, low power consumption, and a small size are desired. Improved illumination performance has become a key goal of many LCD developers and manufacturers in recent times.
A typical backlight module comprises a light guiding plate, a reflector sheet, a prism sheet, a diffusion sheet, and a cold cathode fluorescent lamp (CCFL). The light guiding plate has an incidence interface, an emission interface, and a reflection interface opposite to the emission interface. In order to increase the optical brightness and uniformity of the backlight module, U.S. Pat. No. 5,851,062 discloses a backlight module including a prism sheet on which a periodic pattern is formed. A cross-section of a smallest unit of the periodic pattern is shaped like an M. The M-shape includes a long positive-sloping surface, a short negative-sloping surface, a short positive-sloping surface, and a long negative-sloping surface.
However, the very need for a prism sheet is being increasingly considered to be inconsistent with the ongoing industry trend toward miniaturizing backlight modules and LCD panels and decreasing cost.
U.S. Pat. No. 6,860,753 discloses a light source for a flat panel display device. The light source is generally a tube-shaped lamp. However, light is emitted from the lamp tube in various different directions. Some of the light may be absorbed by a housing of an associated backlight system, or may reach an associated light guiding plate after being repeatedly reflected. These problems may significantly reduce the efficiency of illumination. Indeed, it is widely held that one of the principle shortcomings of a conventional LCD panel is poor light transmission efficiency. That is, a high percentage of light generated by the backlight system is absorbed by various components of the LCD panel. The light transmission efficiency of a typical LCD panel is generally about 5% to 10%. The light transmission efficiency is degraded not only during transmission from the backlight system to the light guiding plate, but also during transmission through attached functional optical sheets such as reflector sheets, prism sheets and diffusion sheets.
What is needed, therefore, is a backlight module which can overcome the foregoing problems and improve illumination performance.